Distant detection of chemical and/or biological warfare materials and contaminates (CBs) is vitally important to joint force and homeland security operations. Once the presence of a CB is detected at a location, preventive measures may be taken prior to entering the area. Intensive research is being conducted to develop systems that are able to perform rapid and reliable detection and identification of CBs at safe standoff distances.
In general, CB standoff detectors receive electromagnetic energy radiated from a target and attempt to identify substances present at the target from characteristics of the radiated energy. Some systems are passive and rely solely on detection of target luminescence and/or absorption/emission spectra in the presence of ambient electromagnetic energy. Other systems actively probe a target with electromagnetic beams and analyze scattering radiant spectra to identify the substances present. While CB detection systems using electromagnetic energy ranging from terahertz and long-wave infrared bands to ultraviolet have been tried, middle infrared (MIR) light has been found to be particularly useful since spectral bands lying in the MIR region can be found in which there are distinctive absorption spectra for CB molecular structures (the so-called MIR “fingerprint”) and in which the natural atmosphere is essentially transparent. A variety of MIR CB sensing systems use scattered light to determine properties of CBs at a distance. While some systems measure only the intensity of backscattered light, others also attempt to collect and analyze polarization states to better characterize CB molecular structures.
One technique to obtain polarization data from backscattered light is differential absorption Mueller matrix spectroscopy (DIAMMS). A DIAMMS-based CB detection system is described in U.S. Pat. No. 6,060,710, issued on May 9, 2000 to Carrieri, et al. (“the '710 patent”), incorporated herein by reference as if fully set forth. In general, a DIAMMS system undertakes identification of unknown substances at a distance by probing a target surface or an aerosol cloud with at least two polarization-modulated middle infrared laser (MIR) beams. One beam (the “excitation” beam) is tuned to a known absorption/extinction wavelength for substances of interest to drive it into molecular vibrational resonance, and a second “reference” beam is tuned to a nearby offset wavelength at which there is essentially no absorption by substances of interest and thus nil resonant molecular vibrations. A detector collects the backscattered polarization modulated radiation or “scattergrams” for both beams. The scattergrams are time varying waveforms which can be Fourier transformed to extract complex frequency components from which excitation and reference Mueller matrices can be derived. A differential Mueller matrix may then be calculated from the excitation and reference matrices and mathematically analyzed to compare it with differential Mueller matrix models of known substances.
Improvements in CB identification by differential-absorption Mueller matrix spectroscopy have been desired. In particular, there is a need for a robust, reliable, field-ready selective CB standoff identification system.